A study of liquid crystalline properties and their relation to the molecular structure of novel ethylene derivatives

Article abstract of DOI:10.1080/15421406.2013.794672

A novel homologous series α-4-[4′-n-Alkoxy benzoyloxy] benzoyl β-4″-methoxy phenyl ethylenes of 12 homologue substances is presented and comprises of 5 compounds (heptyl to tetradecyl) that exhibit both a smectic and a nematic phases. The pentyl, hexyl, and hexadecyl homologues only exhibit the nematic without exhibition of any smectogenic property. The methyl to butyl homologues are not liquid crystalline. All novel liquid crystal (LC) substances of novel series are enantiotropically mesomorphic. The textures of nematogenic homologues are threaded or Schlieren and that of the smectogenic homologues are typical of the smectic A or C. Analytical and spectral data support the molecular structures. Solid-mesomorphic or isotropic, Sm-N, and N-isotropic transition curves of a phase diagram behave in a normal manner. An odd-even effect is exhibited by the Sm-N and N-isotropic transition curves. Thermal stabilities for the smectic and nematic mesophases are 118.0°C and 157.0°C, respectively. The Sm-N mesophase length is between 13°C and 21°C, whereas the N-I mesophase length varies between 20°C and 46°C. The liquid crystal properties of the novel series are compared with known structurally similar homologous series.

Full text of DOI:10.1080/15421406.2013.794672

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A Study of Liquid Crystalline Properties
and Their Relation to the Molecular
Structure of Novel Ethylene Derivatives
R. P. Chaudhari a , A. A. Doshi b & A. V. Doshi c
a Chemistry Department, K. K. Shah Jarodwala Maninagar Science
College , Shri Jagdish Prasad Jhabarmal Tibrewala University
Rajasthan , Ahmedabad , Gujarat , India
b Department of Biochemistry , The Maharaja Sayajirao University of
Baroda , Vadodara , Gujrat , India
c Chemistry Department , Shri Jagdish Prasad Jhabarmal Tibrewala
University , Jhunjhunu , Rajasthan , India
Published online: 03 Oct 2013.
To cite this article: R. P. Chaudhari , A. A. Doshi & A. V. Doshi (2013) A Study of Liquid Crystalline
Properties and Their Relation to the Molecular Structure of Novel Ethylene Derivatives, Molecular
Crystals and Liquid Crystals, 582:1, 63-71, DOI: 10.1080/15421406.2013.794672
To link to this article: http://dx.doi.org/10.1080/15421406.2013.794672
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Mol. Cryst. Liq. Cryst., Vol. 582: pp. 63–71, 2013
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2013.794672
A Study of Liquid Crystalline Properties and Their
Relation to the Molecular Structure of Novel
Ethylene Derivatives
R. P. CHAUDHARI,1,∗ A. A. DOSHI,2 AND A. V. DOSHI3
1Chemistry Department, K. K. Shah Jarodwala Maninagar Science College, Shri
Jagdish Prasad Jhabarmal Tibrewala University Rajasthan, Ahmedabad, Gujarat,
India
2Department of Biochemistry, The Maharaja Sayajirao University of Baroda,
Vadodara, Gujrat, India
3Chemistry Department, Shri Jagdish Prasad Jhabarmal Tibrewala University,
Jhunjhunu, Rajasthan, India
A novel homologous series α-4-[4ꢀ-n-Alkoxy benzoyloxy] benzoyl β-4ꢀꢀ-methoxy phenyl
ethylenes of 12 homologue substances is presented and comprises of 5 compounds
(heptyl to tetradecyl) that exhibit both a smectic and a nematic phases. The pentyl,
hexyl, and hexadecyl homologues only exhibit the nematic without exhibition of any
smectogenic property. The methyl to butyl homologues are not liquid crystalline. All
novel liquid crystal (LC) substances of novel series are enantiotropically mesomorphic.
The textures of nematogenic homologues are threaded or Schlieren and that of the
smectogenic homologues are typical of the smectic A or C. Analytical and spectral
data support the molecular structures. Solid-mesomorphic or isotropic, Sm-N, and N-
isotropic transition curves of a phase diagram behave in a normal manner. An odd–even
effect is exhibited by the Sm-N and N-isotropic transition curves. Thermal stabilities for
the smectic and nematic mesophases are 118.0◦C and 157.0◦C, respectively. The Sm-N
mesophase length is between 13◦C and 21◦C, whereas the N-I mesophase length varies
between 20◦C and 46◦C. The liquid crystal properties of the novel series are compared
with known structurally similar homologous series.
Keywords liquid crystals; smectic; mesogen; mesomorphic; nematic
Introduction
A unique physical state of a matter called the liquid crystalline state, mesomorphic state,
or anisotropic state is neither fully liquid nor crystalline. Thus, not all, but some of the
substances exhibit this intermediate state between crystalline solid and isotropic liquid on
heating. The liquid crystal (LC) state is exploited in many industrial applications. Exhibition
of an LC state by any substance requires suitable magnitudes of anisotropic intermolecular
forces of attractions [1] as a consequence of favorable molecular rigidity and flexibility
[2–4], which arises from the appropriate construction of a molecular structure made up
∗Address correspondence to R. P. Chaudhari, Chemistry Department, K. K. Shah Jarodwala Man-
inagar Science College, Shri Jagdish Prasad Jhabarmal Tibrewala University Rajasthan, Ahmedabad
380008, Gujarat, India. E-mail: info@kksjmsc.com
63

64
R. P. Chaudhari et al.
Scheme 1. Synthetic route to the novel series.
of phenyl rings, a central group or groups lateral and terminal end groups substituted on
phenyl rings. A novel molecular structure consists of three phenyl rings bridged through
–COO– and –CO–CH CH– central groups and n-alkoxy (–OR) and –OCH3 terminal
end groups without any laterally substituted functional group. Thus, the influence of the
–OCH3 terminal end group on liquid crystal properties of the substance can be established,
discussed, and understood.
Experimental
Synthesis
4-Hydroxybenzoic was alkylated by suitable alkylating agents (R–X) to form n-alkoxy
benzoic acids, which on reacting with thionyl chloride yielded the corresponding 4-n-alkoxy
benzoyl chlorides (A) by the modified method of Dave and Vora [5]. α-4-Hydroxy benzoyl-
β-4ꢀ-methoxy phenyl ethylene (B) was prepared by the usual established method [6] from
4-hydroxyacetophenone and anisaldehyde. Components (A) and (B) were condensed in dry
cold pyridine. 4-Hydroxybenzoic acid, 4-hydroxyacetophenone, anisaldehyde, pyridine,
MeOH, KOH, alkyl halides required for synthesis were used as received, except for solvents
that were dried and purified prior to use. The synthetic route to the series is shown in
Scheme 1.
Characterization
A selected number of representative homologues were characterized by elemental analysis,
infrared (IR), and 1HNMR spectroscopy. Microanalysis was performed on a Perkin–Elmer
PE 2400 CHN analyzer as shown in Table 1. IR spectra were recorded on Perkin–Elmer
1
spectrum GX. HNMR spectra were recorded using CDCl3 as solvent. Liquid crystal
properties were determined by an optical polarizing microscope equipped with heating
stage. Textures of the nematic and smectic phase were determined by a miscibility method.

Novel Liquid Crystalline Ethylenes
65
Analytical Data
Table 1. Elemental analysis for methoxy, propoxy, heptyloxy, and tetradecyloxy derivatives
Elements% found (% Calculated)
Sr. no
Molecular formula
C
H
1
2
3
4
C24H20O5
C26H24O5
C30H32O5
C37H46O5
74.28 (74.23)
75.14 (75.00)
76.48 (76.27)
78.12 (77.89)
5.23 (5.15)
5.97 (5.77)
7.01 (6.78)
8.18 (8.07)
Spectral Data
NMR in ppm for Decyloxy Derivative. 0.88 (–CH3 of –OC10H21 group), 1.42 (–(CH2)n–
of –OC10H21), 3.99 (–OCH2–CH2– of –OC10H21), 6.70, 6.77, and 6.93 (–CO–CH CH–
group), 7.84, 7.88, and 7.90 (p-substituted phenyl ring). The NMR spectrum supports the
molecular structure.
NMR in ppm for Dodecyloxy Derivative. 0.89 (–CH3 of –OC12H25 group), 1.26, 1.41
((–CH2)n– –OC12H25 group), 3.99 (–OCH2–CH2– of –OC12H25 group), 6.82, 6.84, and
6.91 (–CO–CH CH–group), 7.84, 7.90, and 7.92 (p-substituted phenyl ring). The NMR
spectrum supports the molecular structure.
IR in cm−1 for Pentyloxy Derivative. 750 (–(CH2)n– group of –OC5H11), 845 (p-substituted
phenyl ring), 1160 (C–O of –OC5H11 group), 1250, 1280, and 1640 (–COO ester group),
920 (–CH CH– group), The IR spectrum supports the molecular structure.
IR in cm−1 for Octyloxy Derivative. 765 (–(CH2)n– group of –OC8H17), 840 (p-substituted
phenyl ring), 1165 (C–O of –OC8H17 group), 1250, 1605, and 1680 (–COO ester group),
850 (–CH CH– group). The IR spectrum supports the molecular structure.
Texture Determination
Hexyl homologue—Threaded nematic
Hexadecyl homologue—Schlieren nematic
Octyloxy homologue—Smectic-A
Results and Discussion
n-Alkoxybenzoic acids are dimeric and their dimerization disappears on esterification
through the breaking of hydrogen bonding between two molecules of n-alkoxybenzoic
acids. Enantiotropic smectic and/or only nematic mesophase formation occurs from the
pentyl to hexadecyl homologues of the novel homologous series α-4-[4ꢀ-n-alkoxy benzoy-
loxy] benzoyl β-4ꢀꢀ-methoxy phenyl ethylenes. The methyl to butyl homologue derivatives
are nonmesomorphic as shown in Table 2. A phase diagram (Fig. 1) shows the phase

66
R. P. Chaudhari et al.
Table 2. Transition temperatures of series in ◦C
Transition temperatures in ◦C
Compound no.
n-alkyl group –CnH2n+1(n)
Sm
N
Isotropic
1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
7
8
10
12
14
16
—
—
—
—
—
—
—
—
—
142.0
132.0
112.0
116.0
112.0
122.0
118.0
116.0
200.0
210.0
196.0
183.0
170.0
175.0
168.0
162.0
145.0
142.0
145.0
149.0
—
106.0
96.0
91.0
105.0
105.0
—
10
11
12
Note: Sm = Smectic; N = Nematic.
behavior through a graph of the number of carbon atoms present in the left n-alkoxy termi-
nal end group versus the transition temperatures as determined from an optical polarizing
microscope equipped with a heating stage. The phase diagram (Fig. 1) consists of three
transition curves: (1) First is the solid-mesomorphic or isotropic transition curve, which
adopts a zigzag path of rising and falling values with an overall descending tendency as
the series is ascended and behaves in the normal established manner. (2) Second is the
smectic–nematic transition curve, which initially rises, passes through maxima at the dode-
cyloxy homologue and then descends at the tetradecyloxy homologue with the exhibition
of an odd–even effect, and behaves in the usual manner. The curve is extrapolated [7] to the
hexadecyloxy derivative of the series, which merges into the solid-nematic transition tem-
perature of hexadecyloxy homologue suggesting that from and beyond the hexadecyloxy
homologue, the smectogenic mesophase formation ceases to appear and hence lamellar
packing of the molecules is absent. (3) Third is the nematic–isotropic transition curve,
which follows a descending tendency as the series is ascended with a negligible rise of
few degrees at the tetradecyloxy and hexadecyloxy homologues and exhibits an odd–even
effect. Thus, all three transition curves behave in a normal established manner. The textures
of the nematic phase are threaded or Schlieren and for the smectic phase of the type A or C.
Variations in mesomorphic properties from homologue to homologue in the present series
are observed. The smectic mesophase length varies from 13◦C to 21◦C and the nematic
mesophase length ranges from 20◦C to 46◦C as shown in Table 2. The smectic–nematic and
nematic–isotropic thermal stabilities are 118◦C and 157◦C, respectively. The smectic and
nematic mesophases commence from heptyloxy and pentyloxy homologues, respectively.
Thus, the present homologous series is predominantly nematogenic and partly smectogenic
with a middle ordered melting type and a short range of liquid crystallinity. The meso-
morphic properties of the novel homologous series are compared with known structurally
similar homologous series. The spectral and analytical data confirm the molecular structure
of the homologues concerned.
The nonmesomorphic behavior of the methyl to butyl homologues is attributed to the
high crystallizing tendency, which arises from the shorter, less flexible n-alkoxy terminal end

Novel Liquid Crystalline Ethylenes
67
Figure 1. Phase behavior of series.
group [1–4]. The pentyl to hexadecyl homologue derivatives are liquid crystalline because
the molecules set themselves at an angle less than 90◦ and resist thermal vibrations exposed
upon them between two temperatures with or without lamellar packing of molecules in
their crystal lattices. All such molecules have an ordered two-dimensional array and exhibit
either/both a smectogenic or/and a nematogenic mesophase [1]. Thus, the molecules of

68
R. P. Chaudhari et al.
Figure 2. Structurally similar homologous series.
the heptyloxy to tetradecyloxy homologues have a layered molecular arrangement due to
the lamellar molecular packing in their crystal lattices from the temperature t1 to higher
temperature t2 enantiotropically and on further continued heating up to a higher temperature
t3, with a statistically parallel orientational molecular two-dimensional order. Thus, the
smectic mesophase formation between t1 and t2 and the nematic mesophase formation
between t2 and t3 occurred. The pentyl, hexyl, and hexadecyloxy homologue derivatives
have an absence of lamellar packing of the molecules with only a statistically parallel
orientational order between t1 and a higher temperature t2, enantiotropically showing only
a nematogenic mesophase without smectogenic mesophase formation. The exhibition of an
odd–even effect and a variation of mesomorphic properties from homologue to homologue
in the novel series is attributed to the sequentially added methylene unit to the n-alkoxy
terminal end group. The overall mesomorphism induced from pentyl to hexadecyloxy
homologue derivatives is attributed to the suitable magnitudes of anisotropic intermolecular
forces end to end and lateral adhesion as a result of varying favorable molecular rigidity
and flexibility [2–4] against exposed thermal vibrations. The mesomorphic properties of
the present homologous series (1) are compared with other structurally similar homologous
series X [8] and Y [9]; as shown in the Fig. 2.
Homologous series 1 and X are isomeric and identical in all respects except for the
position of the –CO– group between the middle and third phenyl rings of the molecules.
The molecules of series 1 and X identically consist of three phenyl rings bridged through
identical –COO– and nonidentical –CO–CH CH– (series 1) –CH CH–CO– (series X)
central groups, with identical left and right terminal end groups. However, they differed in
types of linking groups –CO–CH CH– central bridge for series 1 and X, respectively. The
–CO– group is bonded directly with the middle phenyl ring (series 1) and bonded directly
to the third phenyl ring (series X). Therefore, the net molecular rigidity and flexibility
[2–4], the lateral and terminal end to end anisotropic intermolecular forces of attractions
differ considerably, which reflects in the differing mesomorphic behavior and the degree
of mesomorphism of the homologous series under discussion. Homologous series 1 and
Y are identical to each other except for the right sided terminal end groups –OCH3 and
–H of different polarities, respectively. Therefore their molecular length, length to breadth
ratio, molecular polarities, molecular rigidity and flexibility, end to end attractions, lateral
intermolecular attractions, as well as suitable magnitudes of anisotropic intermolecular
forces of adhesion—which all have direct effect on mesophase formation and degree of
mesomorphism—differ considerably.Table 3 shows the thermal stabilities and commence-
ment of mesophase, etc., for the series under comparison.

Novel Liquid Crystalline Ethylenes
69
Table 3. Average thermal stability in ◦C
Series→
1
X
Y
Smectic–nematic
or isotropic
118.0
(C7–C14)
C7
133.4
(C6–C12)
C6
—
Commencement
of smectic phase
Nematic–isotropic
Commencement
of nematic phase
157.0
(C5–C16)
C5
159.8
(C6–C16)
C6
164.5
(C5–C16)
C5
Table 3 indicates that isomeric series 1 and X differ in their Sm-N thermal stability
by 15.4◦C and N-I thermal stability by 2.8◦C. Second, the smectic mesophase commences
from the heptyl and hexyl homologues, whereas the nematic mesophase commences from
the pentyl and hexyl homologues, respectively for series 1 and X. Thus, the early com-
mencement of the smectic phase and late commencement of the nematic phase are related
to the higher thermal stability (series X), and the late commencement of smectic phase and
early commencement of nematic phase are related to the lower thermal stability (series 1).
The early commencement of the smectic mesophase is related to the extent of molecular
coplanarity [10]. Hence, the molecules of series X are more coplanar than those of series
1. The –CO– group, linking the middle and third phenyl rings bonded through –CH CH–
immediately at the third phenyl ring, gives greater coplanarity than the isomeric series taken
for comparison. The –CH CH–CO– central group maintains more molecular coplanarity
than the –CO–CH CH– central group. Consequently, the lamellar packing of the molecules
in their crystal lattices can be enhanced by the –CH CH–CO– group more than for the
–CO.CH CH– central group. Thus, both series (X and 1) under comparison are closely
similar to each other. Both these central bridges (–CO.CH CH– and –CH CH–CO–) are
comparable with equal length and causes more or less noncoplanarity due to a twist ob-
tained as the oxygen atoms of the –CO– group bump into the nonbonded adjacent hydrogen
atoms [10] of the middle or third aromatic phenyl ring. On account of this difference, the
smectic and nematic thermal stabilities of series 1 are lower than the series X. Hence, the
difference of linking group –CO– at the middle (series 1) or third (series X) phenyl ring in
an isomeric homologous series generates a difference in mesomorphic thermal stabilities
and other mesomorphic properties. The homologous series 1 and Y differ only in their right
handed terminal end groups of –OCH3 and –H, respectively; keeping rest of the molecular
part identical. Thus, the length to breadth ratio, molecular polarities due to high polarity of
the –OCH3 end group (as compared to –H), polarizability, end to end and lateral attractions,
intermolecular closeness, and the suitable magnitudes of anisotropic intermolecular forces
of attractions as a consequence of molecular flexibility [2–4] are higher for the presently
investigated homologous series-1 as compared to series Y. Thus, lower intermolecular at-
tractions of series Y cause a lack of smectogenic character as a result of the absence of
lamellar packing of molecules in their crystal lattices and exhibit only a statistically par-
allel orientational order of the molecules facilitating the formation of only a nematogenic
mesophase from the pentyl to hexadecyl derivatives in series Y. Thus, variations in meso-
morphic properties for the same homologue from series to series is attributed to the varying
lateral and/or terminal end group of different polarity and from homologue to homologue
in the same series is attributed to the sequentially added methylene unit. The absence of
an odd–even effect for the higher homologues from and beyond the decyloxy homologue

70
R. P. Chaudhari et al.
for Sm-N and N-I transition curves of the novel series is attributed to the effects of coiling,
bending, or flexing of the left n-alkoxy terminal end groups or coupling of their longer
n-alkyl chain with the major axis of the core. The observed odd–even effect occurs due to
the progressive and sequential addition of the methylene unit at the left terminal end group
[1].
Conclusions
(1) The novel series of α-4-[4ꢀ-n-alkoxy benzoyloxy] benzoyl β-4ꢀꢀ-methoxy phenyl
ethylenes is predominantly nematogenic and partly smectogenic with a middle
ordered melting type.
(2) The group efficiency order derived for smectic and nematic on the basis of (a)
thermal stability and (b) the early commencement of smectic and nematic phase
are as follows:
(1)
(3) High smectic thermal stability and the early commencement of smectogenic
mesophase-forming tendency are favored more in
bon of in isomeric series.
group bonded to β car-
(4) Variations in mesomorphic properties from homologue to homologue in a same
series and from series to series for same homologue is a direct result of a varying
molecular rigidity and/or flexibility due to varying lateral central or/and terminal
group or groups or/and varying number of aromatic phenyl ring, constituting a
molecule.
(5) Suitable magnitudes of anisotropic forces of intermolecular attractions can induce
mesophase formation.

Novel Liquid Crystalline Ethylenes
71
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